The present invention relates to a humidifier system for humidifying and warming respiratory gases for patients requiring artificial respiratory support.
When a person breathes normally, the mucous membrane of his upper airway can heat and humidify the inspired gases to the body temperature (37° C.) and 100% relative humidity before the gases enter the lung. When a patient is under mechanical ventilation, his upper airway is bypassed due to the artificial airway (such as an endotracheal tube or tracheostomy). Because the gases from mechanical ventilators usually are cold and dry, the patient needs to have the inspired gases heated and humidified before the gases enter into his lung in order not to lose the body heat and water.
Traditionally, there are two techniques to help the patient to achieve the goals of heating and humidifying inspiratory gases from a mechanical ventilator. One of them is to use a device called an active heated humidifier. One example of this device is the heated humidifier developed and manufactured by Fisher & Paykel, a New Zealand based company. This device compromises a heater, a water chamber that is heated by the heater, and sometimes a heated wire that is placed in the breathing circuit of a mechanical ventilator. The cold and dry gases that come from the mechanical ventilator will first flow through the water chamber before entering the patient lung, and will be heated and humidified by touching the hot water in the water chamber. In some cases, a heated wire is placed in the breathing circuit to maintain a stable high temperature in order to avoid water condensation in the long breathing circuit.
The other traditional technique is to use a passive heat moisture exchanger or hygroscopic condenser humidifier (HME). One such HME is made by Hudson RCI Inc. (Temecula, Calif.). This HME is made of a plastic housing and HME material that is placed inside of the housing. The HME material is usually made of hygroscopic foam or paper that may also be treated with salts. When the HME is placed at the out end of the artificial airway, it will retain the heat and moisture from the exhaled gases when the patient exhales. In the next inspiration, the dry and cold gases from the mechanical ventilator will be heated and humidified by the heat and moisture that were retained in the HME in the previous breath.
Active heated humidifiers provide good heating and humidifying capability from the patient physiological point of view. However, it is very cumbersome to use such humidifiers, requiring assembly of the breathing circuit in a special way. It also has poor power efficiency because the heat is wasted through the breathing circuit which is in heat exchange with the room air. The use of heated wire in the breathing circuit may also impose an electrical and fire safety concern. Passive HME, on the other hand, is very convenient for use and does not impose any safety concern from an electrical and fire standpoint. The limitation of HME is its inadequate heating and humidifying power. While the inspired gases need to contain moisture at 44 mg/L in order to get 100% relative humidity at 37° C., a HME usually can only provide 30 mg/L. Therefore, HME is not able to be used in long-term mechanical ventilation due to its inadequate capability.
In order to avoid the shortcomings of the active heated humidifier and the passive HME, some inventors have conceived designs that combine a passive HME with active heating and humidifying elements. Some combined HME and active heated humidifiers are very complicated and have numerous parts, resulting in a high cost. This means that they cannot be used in a disposable manner, and re-use produces a risk of cross infection.
In some combination designs, a HME material, a heating element, and a water evaporating element are all enclosed within a housing. The water evaporating element is connected to the outside water tube to refill water. The heating element is connected to the outside electrical power. All these designs have a risk of electrical and fire hazard. If the heating element is overheated as the result of control error or as the result of empty water tubing, the heating element in the housing may burn the materials that are in the housing. This is risky, especially if the patient is using a high concentration of oxygen.